Upgrade my L-33 to a L-31

Old June 9th, 2018, 11:15 AM
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Upgrade my L-33 to a L-31

I have been trying to find out the differences to do a L-31 buildup from my L-33 that I have for my '70 Cutlass S. Besides the obvious 2 to a 4BBL what are the other differences I need to consider? Yes, I want to keep my 2:56 rear gears and stock torque converter as well as retain the low end torque and highway cruising characteristics. I do have a compatible Quadrajet and a correct casting intake manifold to use. So far, I'm under the impression that the small valve E heads are the same as well as the camshaft profile. Should I take under consideration the stock compression ratio?

Also, since I can't exactly buy a "factory" cam and with today's selections is there really a better recommendation for a "mild" application like this?
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Old June 9th, 2018, 12:22 PM
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The main difference is the carb as you know and the pistons (compression ratio). The 4bbl engine had small dish pistons for 10.25:1 advertised compression ratio and the 2bbl engine had larger dish pistons to achieve 9.0:1 advertised compression ratio.

I would not worry with the compression ratio as 9:1 would work well with today's gasoline.

And yes, you are right about there being better camshafts for a mild application.

Last edited by Fun71; June 9th, 2018 at 12:24 PM.
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Old June 9th, 2018, 06:45 PM
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10.25:1 compression 2bbl 455 in 1970

Originally Posted by Fun71 View Post
The main difference is the carb as you know and the pistons (compression ratio). The 4bbl engine had small dish pistons for 10.25:1 advertised compression ratio and the 2bbl engine had larger dish pistons to achieve 9.0:1 advertised compression ratio.

I would not worry with the compression ratio as 9:1 would work well with today's gasoline.

And yes, you are right about there being better camshafts for a mild application.
Didn't the 310 hp L-33 2bbl used in the SX and optional in Cutlass and Cutlass Supreme feature 10.25:1 compression not 9:1 as used in the B body cars?
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Old June 9th, 2018, 07:36 PM
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Originally Posted by oldsmobiledave View Post
Didn't the 310 hp L-33 2bbl used in the SX and optional in Cutlass and Cutlass Supreme feature 10.25:1 compression not 9:1 as used in the B body cars?
I dug out my Salesmen's brochure as there are some misconceptions regarding the L-33. You are correct about the compression ratio being 10:25:1 but HP rating was 320. The B body Delta 88 Custom and Royale came standard with the 9:1, 310 horse regular fuel engine but was not advertised as an option otherwise.

My L-33 I had rebuilt a number of years ago and has been on an engine stand with the yearly squirting of oil in the cylinders and being turned over. I had installed TRW forged, 10:1 pistons and it has a little over 20K miles on it. I don't think the rings ever seated plus its got a Kenne-Bell cam in it that I dislike. So, I want to refresh it properly to the likeness of the L-31.
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Old June 10th, 2018, 03:45 AM
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320 hp

Originally Posted by marty59 View Post
I dug out my Salesmen's brochure as there are some misconceptions regarding the L-33. You are correct about the compression ratio being 10:25:1 but HP rating was 320. The B body Delta 88 Custom and Royale came standard with the 9:1, 310 horse regular fuel engine but was not advertised as an option otherwise.

My L-33 I had rebuilt a number of years ago and has been on an engine stand with the yearly squirting of oil in the cylinders and being turned over. I had installed TRW forged, 10:1 pistons and it has a little over 20K miles on it. I don't think the rings ever seated plus its got a Kenne-Bell cam in it that I dislike. So, I want to refresh it properly to the likeness of the L-31.
Yes 320 hp on the L-33. My 310 hp posted was a typo.
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Old June 10th, 2018, 08:50 AM
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Originally Posted by oldsmobiledave View Post
Didn't the 310 hp L-33 2bbl used in the SX and optional in Cutlass and Cutlass Supreme feature 10.25:1 compression not 9:1 as used in the B body cars?
I didn't look up specs in a book so I guessed incorrectly that he was working with a low compression 2bbl engine.

For a high compression engine, I would definitely look into an aftermarket cam that would work better with high compression and today's gasoline.

Last edited by Fun71; June 10th, 2018 at 10:22 AM.
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Old June 11th, 2018, 10:32 AM
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So, has anyone rebuilt an L-31? I've been looking at various mild cam selections and it does get confusing although I believe a duel pattern design would be better, but with a small overlap I then have to be concerned about cylinder pressure/compression and pinging. I'd really like to stay around 10.1 and keep my low end torque and power.

I'm still not sure if larger intake valves would be any advantage either, I can go either way with what I have.
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Old June 11th, 2018, 10:55 AM
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Originally Posted by marty59 View Post
So, has anyone rebuilt an L-31? I've been looking at various mild cam selections and it does get confusing although I believe a duel pattern design would be better, but with a small overlap I then have to be concerned about cylinder pressure/compression and pinging. I'd really like to stay around 10.1 and keep my low end torque and power.
I'm still not sure if larger intake valves would be any advantage either, I can go either way with what I have.

I think you should seek the advice of Cutlassefi on this site.

Last edited by OLDSter Ralph; June 11th, 2018 at 11:00 AM. Reason: spelling
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